The present invention relates to biomagnetic field measuring method and apparatus for measuring a biomagnetic field generated by a nerve action of brain as well as a myocardial action of heart of a living body by using a plurality of fluxmeters each consisting of a highly sensitive superconducting quantum interference device (SQUID).
In addition to a magnetic field generated by a current dipole, a magnetic field due to a volume current flowing in the living body is enumerated as a biomagnetic field. Measurement of a normal component (Bz:z component in the Cartesian coordinate system or Br:radius component in the polar coordinate system) is considered to be hardly affected by the volume current. In conventional techniques, the plane of a detection coil connected to a SQUID is disposed in parallel to the body surface to measure Bz or Br which is a normal component vertical to the body surface. Results of the biomagnetic field measurement are displayed in the form of a temporal change waveform of the measured field component or an isomagnetic field map (contour map) for connecting points at which magnitudes of the magnetic field component measured at desired time points are equal to each other. Various analysis methods have been proposed which analyze a magnetic field source participating in generation of the biomagnetic field from the obtained isomagnetic field curve and in typical one of them, analysis is carried out by replacing the magnetic field source with a current dipole.
An isomagnetic field map of a normal component (Bz or Br) of the magnetic field generated by a current dipole is of a pattern having a source pole of magnetic field and a sink pole of magnetic field at positions which are separate from each other from the center where a magnetic field source (current dipole) is positioned. The magnitude, position and direction of the magnetic field source (current dipole) are analyzed in accordance with magnitudes of magnetic field at the two poles and a distance therebetween.
In a first prior art (H. Hosaka and D. Cohen: J. Electrocardiology, 9 (4), pp. 426-432 (1976)), a method is employed for displaying current sources distributed in the myocardium by using an isomagnetic field map of measured normal component Bz with the aim of promoting visibleness of direction and intensity of currents in the myocardium and according to this method, an arrow map is contrived for expressing a current vector J (x, y) defined by equation (1) on measuring points by using an arrow. In the following description, Gothic characters are used to indicate vectors.J(x, y)=(∂Bz(x, y)/∂y)ex−(∂Bz(x, y)/∂x)ey  (1)
In equation (1), ex designates a unit vector in x direction and ey designates a unit vector in y direction. This prior art, however, encounters a problem that when a plurality of current sources exist, it is difficult to discriminate the individual current sources from each other on the basis of the isomagnetic field map of normal component Bz.
In a second prior art (K. Tukada et al: Review of the Scientific Instruments, 66(10), pp. 5085-5091 (1995)), for the sake of visualizing a plurality of distributed current sources, the normal component (Bz or Br) is not detected but tangential components Bx and By are measured by using a detection coil whose plane is disposed vertically to the body surface. Each of the measured tangential components Bx and By is displayed in the form of an isomagnetic field map. The tangential components Bx and By measured according to the second prior art are considered to be affected by the volume current but in an isomagnetic field map of two-dimensional vector magnitude Bxy obtained by synthesizing Bx and By measured at time point t pursuant equation (2), a peak can always be obtained directly above a current dipole and therefore, even when a plurality of current dipoles exist, individual current dipoles can be separated for visualization.|Bxy(x, y, t)=√{square root over ( )}{(Bx(x, y, t))2+(By(x, y, t))2}  (2)
In a third prior art (Y. Yoshida et al: Tenth International Conference on Biomagnetism, Santana Fe, N.Mex., Feb. 17 (1996)), a normal component and two tangential components of a biomagnetic field are detected by using a vector magnetic field sensor consisting of three detection coils having coil planes which are orthogonal to each other, detection results of the magnetic field components are converted in terms of the Cartesian coordinate system to determine Cartesian coordinate system components Bx, By and Bz, and an isomagnetic field map of the normal component Bz and an isomagnetic field map of two-dimensional vector magnitude Bxy are displayed, respectively.
In a fourth prior art (K. Tsukada et al: Tenth International Conference on Biomagnetism, Santana Fe, N.Mex., Feb. 17 (1996)), two tangential components Bx and By of a biomagnetic field are detected and an isomagnetic field map based on |BXY|=|Bx+By| is compared with an isomagnetic field map based on a normal component Bz.
Available as diagrams for indicating measurement results of electrical physiological phenomena in a living body are a magnetoencephalogram (MEG) obtained through measurement using a magnetoencephalogram and an electrocardiogram (ECG) obtained through measurement using an electrocardiograph. In measurements of the electrocardiogram, a body surface potential map for mapping an electrocardiographic figure by using a plurality of electrodes is of a well-known technique. The MEG or the body surface potential map is depicted in the form of an isopotential map for connecting isopotential points.
In a fifth prior art (T. J. Montague et al: Circulation 63, No. 5, pp. 1166-1172 (1981)), an isointegral map obtained by integrating a temporal change waveform of an output of each one of a plurality of electrodes over a desired time interval is depicted as a body surface potential map.
In the following description, “biomagnetic field” means “magnetic field generated from a living body”, “cardiac magnetic field measurement” means “measurement of a magnetic field generated from the heart”, and “cardiac magnetic wavaform” means “waveform indicated by a magnetocardiogram (MCG) obtained through cardiac magnetic field measurement”. Further, “encephalic magnetic field measurement” means “measurement of a magnetic field generated from the brain” and “encephalic magnetic waveform” means “waveform indicated by a magnetoencephalogram (MEG) obtained through encephalic magnetic field measurement”.
Each of the conventional isomagnetic field maps of the respective components has inherent features. In the presence of a single current dipole, the position, magnitude and direction of a current source can be analyzed with ease by using the isomagnetic field map of normal component Bz. On the other hand, the isomagnetic field map of two-dimensional vector magnitude Bxy obtained from measurement results of tangential components Bx and By features that even in the presence of a plurality of current dipoles, individual current dipoles can easily be discriminated from each other. But, for detection of a magnetic field, coils are required to be provided in x and y directions and the number of coils is doubled as compared to detection of only the normal component Bz. In vector measurement for measuring all the components Bx, By and Bz, the number of required coils is tripled as compared to detection of only the normal component Bz. Accordingly, the magnetic field sensor consisting of a detection coil and a SQUID is increased in number and in addition, the signal processing circuit and the like are also increased in number, raising a problem that the biomagnetic field measuring system becomes an expensive one. Further, the first prior art is disadvantageous in that arrows are merely indicated on measuring points and detailed distribution states of current sources are hardly discriminated.
From the isomagnetic field map indicated in terms of a biomagnetic field component, the position, magnitude and direction, at a desired time point, of a current source in a living body can be analyzed and detailed information about changes in position, magnitude and direction of the current source can be known. Conventionally, dynamic changes in various kinds of information pieces are captured by using many figures displayed on or delivered to the apparatus so as to diagnose a disease. In the prior arts, however, many diagrams or maps indicating various kinds of information pieces are needed for diagnosis and abnormality of changes in various kinds of information pieces is known empirically. As will be seen from the above, in the prior arts, the processing of displaying, on a single map, systematic information as to what magnitude of current flows through which portion of a living body and as to which region an abnormal bio-current passes through is not executed. In the case of the body surface potential map, an isointegral technique was reported. This isointegral map was drawn by connecting between the same integral values over a desired time interval (for example, an interval during which waves of Q, R and S are generated and an interval during which S to T waves are generated). The advantage of this isointegral map is that information of the heart can be obtained from only a single electrocardiographic figure. But, in the isopotential map when the current source in the heart is assumed to be a single current dipole, a figure results disadvantageously in which an positive peak and a negative peak do not exist immediately above the current dipole but exist at a position which is separate from a point immediately above the current dipole. Further, when the position of the current dipole remains unchanged but the direction of the current dipole changes, the anode and cathode peak positions change, raising a problem that when potential is integrated, correspondence between the current source and the peak of an integral value is impaired. Like the case of the electrocardiogram, mere integration of a component of a biomagnetic field obtained through biomagnetic field measurement faces a problem that the peak position of the biomagnetic field component does not correspond to the position of the current source. Further, with only the isointegral map obtained from the electrocardiogram, because of the individual difference such as the position and size of the internal organs, making it difficult to accurately determine abnormality such as a disease by simply gathering from the isointegral map.